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Friday, October 11, 2013

Protecting Your DUT During Test with Logical Trigger Expressions

In this post we will talk about what logical trigger expressions are and how they can be used to help protect your DUT from damage during test. Logical trigger expressions are a new capability that is starting to appear in modern high performance system power supplies. This capability moves logical decision making based off of trigger signals from the test software into the test hardware. By moving this decision making from software to hardware we speed up the reaction time to a potentially dangerous situation that may occur during test. This increase in reaction time could be the difference between no damage and catastrophic damage to a high value DUT.

Logical trigger expressions allow you to "and," "or," and "not" multiple trigger conditions in an instrument together to create an expression based off of triggers and logic gates. If the output of the logical expression becomes true the instrument will initiate some type of user defined response, such as shut off the output. The value of this capability is it allows your test system to react in hardware time instead software time to a potentially damaging condition to your DUT. Let's take a look at an example test scenario where logical trigger expressions can be a valuable tool for protecting your DUT.

In our scenario we are testing a high value DUT. We want to monitor the DUT's current draw and its temperature at various points during testing. If the current moves out of a specific window (meaning a high limit as well as low limit) or if a temperature point goes over a set value we want the power supply to shut off its output. We will look at two methods to address this scenario, one that uses a power supply with logical trigger expression capability and one that does not so we have to do it in the test software.

Addressing the scenario with logical trigger expressions
For this example we will use Agilent's Advanced Power System N7953A Dynamic DC Power Supply, which has the logical trigger expression capability built-in. For the temperature measurements we can use any multichannel DAQ unit with the capability to send a trigger out when a pre-defined temperature level is detected on one of the channels, such as the Agilent 34972A DAQ / Switch Unit. The Advanced Power System has free software available that makes it easy to create logical trigger expressions with a graphical user interface. Below is a screen shot from the software of the logical trigger expression created for the N7953A to address our scenario.

The "CL+" icon defines the upper current limit of our window, the "CL-" icon defines the lower current limit of our window, and the "Digital Pin1+" icon is the external trigger signal from our temperature measurement instrument. With these three conditions connected to an "OR" gate, if one is triggered (becomes true) the output of the OR gate becomes true and will turn the output of the power supply off. Once the logical trigger expression is defined it can then be downloaded to the N7953A or we can get the corresponding SCPI commands generated from the software to cut and paste into our test software. With this setup in the power supply hardware we can now react to an unsafe condition in test and turn off the power supply output in < 1ms.

Addressing the scenario without logical trigger expressions
To address this scenario without logical trigger expressions the most straight forward way would be to have the test system monitor the current and temperature by polling the DAQ and the power supply for the temperature and current measurements. Once an out of range condition is detected the software can then tell the power supply to shut off its output. The problem with this approach compared to using a logical trigger expression is reaction time. If we take into account the IO latency, the instruments command processing time, and the program's response time (assuming we are not working with a real time operating system) we are looking at a reaction time of ~10 to 100ms, which is 10 to 100 times slower compared to using a logical trigger expression. Of course we could speed this up by adding some custom external hardware to monitor the conditions and give a hardware time response, but this would add a lot of complexity to the test.

The example scenario we looked at for using a logical trigger expression was fairly simple. Complex expressions can be setup with multiple layers of logic gates if needed. Also the result of the expression can range from turning off the supply output (like we did in the example scenario) to sending a trigger out of the power supply to some custom user defined action.

In this post we looked at a new trigger capability that is beginning to appear in modern high performance power supplies, logical trigger expressions. This capability can be extremely valuable when testing high value DUTs becuase it can greatly increase the reaction time to a potentially damaging condition that occurs during test. If you have any comments on this post please share them below and if you have any questions feel free to email me.

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